1. Field of the Invention
The present invention .relates to agricultural mineral nutrients and methods for their preparation. More particularly this invention is directed to a new controlled slow releasing potassium divalent metal phosphate nutrient composition. This invention provides a new and effective method of preparing the new composition in the form of hard granules from caustic potassium, divalent metal oxide powders, and acid phosphates.
2. Description of the Prior Art
Potassium is one of the three plant macronutrients. Like nitrogen, moderate amounts of potassium can cause foliage and root burns on plants. Because of the high water solubilities of most potassium salts used as plant nutrients, much of the potassium supplied as fertilizer is lost to leaching by rain or irrigation water.
Conversely, potassium is fixed so strongly in some water insoluble compounds that it does not solubilize, decompose, or release at rates fast enough to allow appreciable assimilation by plants even in a full growing season. Potassium metaphosphate, potassium aluminum silicate, and the naturally occurring micas are examples of potassium compounds which supply little potassium for assimilation as a plant nutrient in a single growing season.
There has been no success reported in the prior art in preparing controlled slow release potassium plant nutrients by chemical reaction of soluble potassium compounds. The current state of the art requires sulfur or resin coating of soluble potassium salts to provide controlled slow release plant nutrient.
The compound potassium magnesium phosphate in the prior art has been reported as source of water insoluble potassium. Some prior art sources also report potassium magnesium phosphate as water soluble potassium. The prior art has reported the water insolubility of similar compounds wherein the magnesium moiety was substituted for by manganous, ferrous, cupric, cobaltous, and zinc moieties.
Bassett and Bedwell, described the early preparation of potassium metal phosphates in "Studies of Phosphates--I," Journ. Chem. Soc. (London) 1933, 854-871. They reacted aqueous soluble divalent metal salts with a large stoichiometric excess of dipotassium phosphate and dried the resulting product.
In U.S. Pat. No. 3,285,731, Salutsky et al disclosed the preparation of complete particulate solid fertilizers which were long lasting and non-burning and consisted essentially of magnesium ammonium phosphate blended with between 1 and 99 percent potassium magnesium phosphate, and taught that potassium requirements of plants could be supplied over a long period of time without burning by potassium magnesium phosphate. The magnesium ammonium phosphate and potassium could be commingled and granulated or could be coprecipitated to form the mixture of the two compounds which was found to be necessary for the successful functioning of the Salutsky composition. They found that the higher the ratio of magnesium potassium phosphate, the more controlled was the release of nitrogen, and that the release of potassium was controlled by the amount of magnesium ammonium phosphate contained.
A free flowing granular, non-burning, non-crumbling fertilizer composition, consisting essentially of 55.5-69 percent coprecipitated and cogranulated magnesium ammonium phosphate, 6-8 percent magnesium potassium phosphate, 5-8 percent monoammonium phosphate, 13-20 percent monopotassium phosphate, 4-6 percent magnesium hydroxide, and 0.9-1.8 percent urea, was disclosed by C. C. Legal et al in U.S. Pat. No. 3,585,020. The product was prepared by mixing aqueous urea and ammonia, aqueous magnesium hydroxide slurry and blending in a Pugmill with aqueous phosphoric acid, aqueous potassium hydroxide, water and recycled solids with water concentration maintained between 25 and 35 percent by weight. The wet fertilizer granules withdrawn from the Pugmill were then dried with air at a high temperature between 460.degree. and 650.degree. F.
In Japanese Patent SHO 46-24047, A. Akiyama et al produced a fertilizer by reaction and precipitation of magnesium hydroxide, ammonium phosphate, and potassium compounds and obtained largely water insoluble nitrogen and water soluble potassium products.
Rothbaum reported in the New Zealand Journal of Science, 1973, Vol 16, 539-552, the preparation of potassium magnesium phosphate hexahydrate by a precipitation method of reacting potassium hydroxide with magnesium phosphate, which was derived in fair yield from disodium phosphate and magnesium sulfate.
From a survey of the prior art, it may be seen that potassium divalent metal phosphates have been known for many years, and that the compounds contain the two nutrients most needed with slow controlled release properties for fertilizers. However, to this date no quantitative control of the release of potassium, phosphorous, and divalent metals has been achieved. Therefore no commercialization has been made of this material which offers great promise in the field of agricultural nutrients.
The prior art has supplied no teachings regarding quantitative control of the nutrients potassium, divalent metals, and phosphate from a single compound and has reported generally slow release of potassium, only from plant food mixtures, particularly from potassium magnesium phosphate admixed with magnesium ammonium phosphate.
The preparation methods for the potassium divalent metal phosphates utilized in the prior art consisted of precipitation from aqueous solution or reaction in the form of a thick mass of solids and liquids including substantial recycle of solids in a Pugmill, followed by granulation in a rolling bed.
Although the prior art cited above discloses water insoluble phosphate and divalent metal nutrients, no teachings are made of compositions in which these nutrients are water insoluble, yet controlled slow releasing. Also not reported are the properties which a composition must possess to exhibit the desired controlled slow and complete release of these nutrients. The prior art has taught no methods for preparing these new compositions.
The prior art provides analytical methods for defining slow and controlled release nitrogen concentrations and for nitrogen Activity Indices. Official methods provided in the Official Methods of Analysis, 15th Edition, of the Association of Official Analytical Chemists, Volume 1, relating to this disclosure are listed as follows: 945.01 Nitrogen (Water Insoluble), 955.05 Nitrogen Activity Index, 955.04C Nitrogen (Total), and 983.02 Potassium (Total). No official methods have been provided for the determination of water insoluble potassium and potassium activity index.
For determining Cold Water Insoluble Potassium (CWIK), Hot Water Insoluble Potassium (HWIK), and Potassium Activity Index (KAI) in this disclosure, the nitrogen system was adapted for use with potassium. In this adapted procedure, total potassium was determined by Method 983.02, and the CWIK by 945.01, except that potassium was determined instead of nitrogen. HWIK and KAI were determined by the procedures of Method 955.05, except that potassium was analyzed instead of nitrogen. The Activity Index is sometimes referred to as Availability Index and is indicative of the availability of the water insoluble nutrient to a crop in a growing season. A 100 index indicates complete availability of the cold water insoluble nutrient in one growing season, and a 0 index indicates no availability in one season.
Prior art has provided in the above mentioned Edition Method 958.01 for determining total phosphorous, Method 970.01 for water soluble phosphorous, and 963.03 for citrate insoluble and soluble phosphorous. Citrate solubility is indicative of phosphate availability in the soil.
Official Method 965.09 provides a method of determining minor nutrients in fertilizers and specifically the divalent metals calcium, copper, iron, magnesium, manganese, zinc, and cobalt. Availabilities in the soil of these metals may be determined by measurement of the solubility of the metals in ethylenediaminetetraacetic acid (EDTA).